1. Field of the Invention
The present invention relates to communication systems, and, in particular, to controlling channel assignments in a communication system, such as a hybrid fiber/coax (HFC) communication system.
2. Description of the Related Art
A network interface unit (NIU) is a component of a hybrid fiber/coax communication system that converts radio frequency (RF) signals over a coax cable into telephony and video signals. In an HFC communication system, communication signals are conveyed between a central office (CO) and an array of NIUs over a combination of linear lightwave networks (e.g., over optical fibers) and coaxial cable networks. The CO contains a host digital terminal (HDT) and other communications equipment, such as video amplifiers, receivers, and transmitters. Communication signals originating from backbone network sources (e.g., interoffice networks, CATV, video, and other broadband services) external to the CO are sent to the HDT and the other equipment at the CO. These signals are formatted and transmitted to the NIUs by the HDT and the other equipment at the CO in accordance with a communication protocol and format being followed by the HFC communication system.
The NIUs are electrically connected to and communicate with customer premises equipment (CPE). CPE may include such devices as telephones, computers, television receivers, and other communication equipment. The NIUs process the messages and communication signals from the CO, format the signals, and transmit the signals to the proper customer premises equipment. The NIUs also transmit responses to the HDT as well as unsolicited messages (e.g., off-hook indication) in accordance with the system protocol. Most of the communication signals and all of the messages received by the NIUs are transmitted by the HDT. In addition to transmitting communication signals to NIUs throughout the network, the HDT also receives messages from the NIUs. The HDT must be able to identify the type of the received signal, associate that signal with a particular NIU, transfer the received communication signal to the proper external network or service, and, if necessary, properly respond to the particular NIU that sent the signal. The HDT must also be able to detect new NIUs that have been added to the communication system and assign identification numbers to the new NIUs.
FIG. 1 shows a generic hybrid fiber/coax communication system 10. HFC system 10 comprises at least one RF distribution shelf (RFDS) 12 and an array of network interface units 14 interconnected by hybrid fiber/coax access network 16, which provides RF channels between them. An RFDS is a set of hardware components, typically located on the same shelf of a rack of communication equipment, that converts incoming and outgoing signals between the digital and RF domains. RFDS 12 is located at a centralized place such as a telephone company central office or a cable access television (CATV) company headend. Typically, each RFDS serves a large number of NIUs (e.g., 500 NIUs). The NIUs can be located outside a home, on a utility pole, or in the basement of an apartment to serve one or more customers. Each NIU is capable of converting signals generated by customer premises equipment (CPE) 18, such as a telephone set, into appropriate RF signals to transmit to the RFDS. The RFDS in turn converts the RF signals into an appropriate format to be transmitted to the backbone networks.
The HFC system provides downstream RF channels (i.e., RF channels from the host digital terminal to the customers) from each RFDS to its subtending NIUs and upstream RF channels (i.e., RF channels from the customers to the host digital terminal) from the NIUs back to the RFDS. In the upstream direction, the NIU converts signals generated by customer premises equipment, such as a telephone set or a PC, into RF signals appropriate for communication with the RFDS. The RFDS in turn converts the RF signals into appropriate format for transport through the backbone networks. In the downstream direction, the inverse process takes place.
In the above model, each RFDS has many upstream RF channels available for its subtending NIUs. At any given point in time, the RFDS might be using only a subset of the available upstream channels as working channels to carry user traffic, with the remaining upstream channels maintained as spare channels. The RFDS continuously monitors the health of the working channels and spare channels in the background. When the RFDS detects a transmission failure on a working upstream RF channel due, for example, to unwanted interference such as ingress noise, the RFDS needs to coordinate actions with the subtending NIUs to switch the failed working channel with a healthy upstream spare channel. This switching needs be completed as quickly as possible to minimize service interruption.